Interpretive Summary: Free air carbon dioxide enrichment systems have been used to expose crop plants in field plots to elevated carbon dioxide concentrations in order to evaluate crop responses to projected increases in atmospheric carbon dioxide. However, free air carbon dioxide enrichment systems also expose plants to pulses of carbon dioxide concentration considerably above the target concentration, in addition to increasing the mean concentration. This work found that pulses of high carbon dioxide concentration typical of those in free air carbon dioxide enrichment systems caused large and persistent decreases in the water loss rate and the carbon dioxide uptake rate of leaves. This work will be of interest to scientists evaluating crop responses to rising atmospheric carbon dioxide.

Technical Abstract:
Free air carbon dioxide enrichment (FACE) systems typically expose plants to pulses of carbon dioxide concentrations considerably above the control set-point, in addition to increasing the mean concentration. The importance of such pulses of higher carbon dioxide concentrations to plant function is unknown. I exposed single leaves of wheat and rice plants to pulses of elevated carbon dioxide similar to the frequency, duration, and magnitude of pulses observed in FACE systems. Leaves were exposed for 30 minutes to pulses with magnitudes sufficient to bring the mean carbon dioxide concentration 45 ppm above the background concentration of 515 ppm. Stomatal conductance began to decrease within a few minutes of exposure to once per minute pulses of high carbon dioxide concentration, in both species. Both species had 20 to 35% lower stomatal conductance for at least 30 minutes after the termination of the pulses. After the pulses had stopped, photosynthesis was in all cases 10 to 35% below the values expected for the observed sub-stomatal carbon dioxide concentration, which suggests that either patchy stomatal closure occurred or that photosynthesis was directly inhibited. It was also found that a single pulse of elevated carbon dioxide concentration reaching a maximum of 1000 ppm decreased stomatal conductance in wheat and rice on average by 20 and 17%, respectively at 10 minutes after the pulse. Based on these results, it seems likely that plants in many FACE systems, and possibly also other systems for growing plants at elevated carbon dioxide, have lower photosynthesis and stomatal conductance than would plants exposed to the same mean carbon dioxide concentration but without pulses of higher concentration.